Advances in pine weevil resistance research may result in new weapons to battle the bugs

The Swedish forestry is a huge industry with a disproportionate vast global impact. Although Swedish forests comprise less than one percent of the world’s total forested area, they produce some 10 % of its yearly output of lumber and paper mass. That is partly due to a highly developed infrastructure, and party due to very efficient ways to conduct forestry. The most production-focused form is the clear-cutting forestry, which has been the dominating practice in all Swedish forests since the 1950’s.

The most important aspect of this practice is that all trees in a given stand is of the same age, and preferably (but not necessarily) of the same species. This means that the entire stand may be harvested at the same time, and also grows faster, since the larger trees don’t shade the smaller ones. The opposite of clear cutting is various kinds of selection forestry, in which the forest consists trees of all ages (another name is uneven-aged forestry), and only a selection is harvested. The economic output of these respective practices differ vastly. Studies show that clear cutting forestry increases productivity by 30 % in comparison to selection forestry, and that harvesting costs decrease by 28 % and fuel consumption by 20 %. Between these two extremes lies a wide spectra of other practices which to a varying degree employs so called continuous cover forestry, i.e. by various means avoids creating large clear cuts.

The pine weevil threatens replanting

However, the economic efficiency of clear cutting forestry does not come without other costs. One such is that the practice makes itself vulnerable to a plethora of pests and pathogens. This is particularly true regarding fungi and insects that are specialized on niches that are rare in natural forests (and which they therefore must dominate) but become very common during clear-cutting forestry. One such insect is the pine weevil (Hylobius abietis).

The pine weevil is an insect that is specialized on the bark of newly formed stumps or broken branches of conifers. It places its eggs on the roots of the stump or in the soil right next to it, and when they hatch, the larvae eat the bark. While the tree is still alive it can fight off the larvae and when the stump has been dead for a few years the bark is no longer usable as food. That is why the weevil exclusively needs recently dead trees, such as those broken in a storm, or large branches that have been snapped by animals or falling trees. Since such resources are rare in a natural forest, the pine weevil must be very adept at finding them, and has developed a very keen sense of smell for freshly broken wood.

The availability changed of course drastically once man started to use the forest for trees. Weevils have always been drawn to our harvest sites, but it is the clear-cutting revolution that has caused the problem to grown into the avalanche it is today. For a bug that can smell a fallen tree miles away, a several hectares large clear cut is like a beacon in the night.

Dangerous early years

During the first summer after harvest, a large amount of weevils will be drawn to the clear cut, to mate and lay their eggs. When they arrive, they are usually not yet sexually mature, and for that they must eat. Their favorite food is young, green bark, such as the fresh shoots of a fully grown tree, but if the next generation trees have already been planted, the pine weevils will happily feed on them as well. If the plants are less than one centimeter in diameter, it is likely that the attack will lead to them be girdled and killed.

The newly arrived weevils will lay their eggs the whole summer, to be hatched 1–2 years later. Therefore, the year after the harvest is also dangerous to new conifer seedlings. The relatively fresh stumps will still attract weevils from afar during the spring and early summer, and when autumn comes, the first eggs from the year before will hatch. The emerging bugs are not yet able to fly and has to feed on the smallest, and most vulnerable, plants. They will hibernate in the soil over the winter and keep feeding in the spring, when even more eggs will hatch.

Huge costs

The attraction of the clear cut is so strong that a single hectare may give rise to 100 000 new weevils within two years. Usually, 2 000 – 2 500 seedlings are planted per hectare, and if no particular measure is taken to protect them it is very conceivable that every single one will succumb, at least in southern Sweden. The risk decrease the further north the stand is located, but even along the coastline of Norrland, half the plants may be lost. The attacks cause losses of hundreds of millions SEK every year, and thus it is of utmost importance to prevent the bugs from feeding on the newly planted trees.

Many methods have been tried to achieve this. In the 19th century, the entire stump were sometimes removed to prevent the weevils from laying eggs, but such methods would be very costly in large-scale production systems. In the 1930’s, it was understood that the old stumps only remain attractive to the bugs for two years after harvest, and once fully developed weevils have left the site two years later, the risk of attacks is small. Allowing the clear cut to rest is still today a viable option to reduce the losses. But the longer the rest, the stronger the competition from other vegetation that flourishes once the large trees are gone. Also, every lost year of growth will translate to less income in the end. According to Swedish forestry law, the site may rest for no more than three years after harvest.

Chemicals and barriers

In the 1940’s, the chemical compound DDT was invented. It had a huge impact on all sorts of insect control. As we know today, and were not unaware of back in the day either, it has a devastating impact on in particular birds and fishes. In 1975, all usage was banned in Sweden. Since then, other chemicals has been developed and used in the belief that they are harmless, only to eventually be banned once other, unforeseen consequences for the environment and human health has been discovered.

Tilling, or other types of soil scarification, can significantly reduce the losses. This was discovered in the 1970’s and is still today a very important counter measure. The idea is to remove the soil layer in order to plant in the exposed the mineral earth. The young tree suffers less competition from other vegetation, it can establish itself faster and reach a weevil-safe size sooner, and the weevils do not gladly walk across the mineral soil, at least not if the distance is further than 10 centimeters.

In the 1980’s and -90’s, development of new methods gained momentum. This resulted in so-called barrier protections and protective coatings. The idea is to put some kind of “sock” of paper or plastics around the base of the plant, or to coat them in a covering substance of wax, latex or sand. The coating is more suitable for mass production, but today there are several commercially available alternatives based on both principles.

And this should be cause for celebration, because in October 2023, all sorts of chemical control of weevils in Sweden will be prohibited. Fortunately, both the barrier and the coating methods is just as efficient as the insecticides, and a nice example of how environmentally friendly alternatives may arise without increased production costs or loss of value in the end.

Weevil-controlling forestry

There are also some forestry-based methods that may decrease losses. If a screen of adult trees are left at harvest, at least 100 per hectare, feeding on the young plants will decrease, in particular feeding by the flying weevils that are drawn to the site from afar. They eat shoots on the screen trees instead of on the vulnerable young plants. The adult trees also shade the ground, which decreases weevil activity. Once the plants reach a diameter of one centimeter, the screen can be removed. Another method is to use older plants, which already from the start are large enough to withstand weevil feeding. Curiously enough, similar results have been achieved using the opposite approach, with so called mini-plants. They are yet too small to be attractive to the weevil, and once they reach a size suitable for feeding the bugs have already left the site.

Some 80 % of all Swedish forests are regenerated through planting. But natural generation through mother trees also decreases pine weevil losses. This is probably because plants that have grown from seeds in the soil are more vital than those that are planted, and may produce more resins. They will be attacked equally often, but the bugs need to move around a lot and make new attacks because of the resin flow, which decreases the risk for girdling.

Important to adjust

Thus, a wide array of effective counter measure are available today, and what works best is highly dependent on the geographic location of the stand and its environmental variables. For example, a dry site is more exposed to weevil feeding than a wet, and shortly after a forest fire the risks are even larger, probably because the insects have even fewer feeding alternatives. The best possible protection may be obtained by waiting 2-3 years after harvest, till the soil vigorously, and then plant at least 2-year-old plants with protective coating, but each such measure also means a higher cost (such as larger plants being more expensive and harder to come by than smaller). The number, and choice, of control measures must be considered with the assessed risk for attacks in mind, based on the nature of the stand in question.

However, weevil research is not static, and progress is made continually. In her thesis from SLU, Yayuan Chen has investigated a novel method to protect the plants.

Tricking plants to defend themselves

Instead of physically preventing the pest from reaching the plant, this method uses the plants own defense mechanisms. When a plant is attacked by a feeding insect or a fungus it will defend itself, by for example producing antimicrobial substances or making its tissues less tasty. The protection is efficient but expensive, and for that reason not activated until needed. The activation is also not instantaneous, and the pest or pathogen may cause significant damage before the system is fully effective.

Yayuan Chen’s method aims at tricking the plants they are under attack, and in so doing initiate their defense mechanisms already before a would-be attacker starts to feed on it. This can be achieved in several ways. The most straightforward is to simply physically harm the plants, but there are more efficient alternatives. When the plant is wounded, it produces a plant hormone called methyl jasmonate (MeJA), which triggers the defense system. Yayuan Chen studied whether the defense may be initiated by an external treatment of MeJA, and if so, whether the effect was stronger or weaker than by mechanical wounding. The method itself is not novel: in earlier studies, pine weevil damages decreased by 80 % in treated plants compared to untreated. But it is not yet practically useable, and one of the hurdles is knowing when, and to what extent, the treatment should be applied. This is what Yayuan Chen’s thesis focuses on.

Treatment in nurseries

For practical reasons it is important to know for how long after a MeJA-treatment the plants are protected, and when the treatment will be the most effective. The majority of plants to be used on Swedish clear cuts are packed in boxes in the nurseries during autumn and subsequently stored in freezer rooms until they are to be planted, usually in the spring or early summer. In order for the treatment to be a practically feasible method, it is highly preferable that it can be conducted in the nursery before winter storage and still be effective during planting the next year.

Yayuan Chen’s results showed that a MeJA-treatment in the fall resulted in half as severe weevil attacks during the coming season. They also showed that a second treatment after storage conferred no extra effect, and that the treatment is efficient for at least a year.

But the reason the plants do not keep their defense turned on all the time is that it costs energy, that otherwise would have been used for growth. Yayuan Chen’s studies confirm that treated plants lose growth, and that they lose more if the MeJA-dose was larger. But the resistance to attack does not increase in a similar fashion. Thus it is important to apply an optimal dose, maximizing protection without losing more growth than necessary.

Breeding for resistance

Yayuan Chen also found that minor wounding of the plant may confer similar protection as MeJA-treatment. She compared the effect of MeJA to that of small needle piercings, a cut small window in the bark, and a larger window. The piercings led to even worse attacks than untreated plants and the larger windows gave no effect at all, but a small window initiated as strong a defense reaction as did the MeJA-treatment. It also had the benefit of not causing growth loss. The results thus show that mechanical wounding theoretically could be used to kick start pine weevil resistance, but also that it might be difficult to cause wounds of optimal size, and that it would be a far more difficult process to administer than just spraying a MeJA-suspension on plants packaged in boxes for winter storage.

Besides the protection that is triggered by an ongoing attack, plants also has a so called constitutively expressed protection, that is, a continually activated defense. Yayuan Chen wanted to examine if it would be possible to breed conifer plants based on this trait, and had access to ongoing breeding programs for spruce trees. These programs aim at breeding for fast growing trees, but Yayuan Chen discovered that they also carry a variation in resistance towards weevil attack. Theoretically, it should be possible to breed for this trait as well. Unfortunately, the trait had low hereditability, meaning it is not certain that the progeny will inherit the resistance even if it was present in the parents.

Writer: Mårten Lind.